sync is a standard system call in the Unix operating system, which commits all data from the kernel filesystem buffers to non-volatile storage, i.e., data which has been scheduled for writing via low-level I/O system calls. Higher-level I/O layers such as stdio may maintain separate buffers of their own.
As a function in C, the sync() call is typically declared as void sync(void) in <unistd.h>. The system call is also available via a command line utility also called sync, and similarly named functions in other languages such as Perl and Node.js (in the fs module).
The related system call fsync() commits just the buffered data relating to a specified file descriptor.[1] fdatasync() is also available to write out just the changes made to the data in the file, and not necessarily the file's related metadata.[2]
Some Unix systems run a kind of flush or update daemon, which calls the sync function on a regular basis. On some systems, the cron daemon does this, and on Linux it was handled by the pdflush daemon which was replaced by a new implementation and finally removed from the Linux kernel in 2012.[3] Buffers are also flushed when filesystems are unmounted or remounted read-only,[4] for example prior to system shutdown.
Some applications, such as LibreOffice, also call the sync function to save recovery information in an interval.